Add support for BookKeeper indexDirectories configuration

[Denovo1998]

Motivation

When running BookKeeper with DbLedgerStorage, RocksDB index placement (indexDirectories) can have a significant impact on IO behavior and tail latency. The current Helm chart does not provide first-class support for configuring indexDirectories or mounting a dedicated volume for it, so index and ledger data often end up sharing the same storage unless users manually customize templates, increasing operational overhead and limiting performance tuning.

Modifications

• Added a pulsar.bookkeeper.indexDirectories helper that supports both string and string[], and defaults to bookkeeper.volumes.index.mountPath when an index volume is enabled and no explicit value is provided.
• Injected indexDirectories into the rendered BookKeeper configuration when applicable (also setting PULSAR_PREFIX_indexDirectories).
• Updated the BookKeeper StatefulSet startup logic to mkdir -p for all configured indexDirectories paths to ensure they exist at runtime.
• Introduced an optional bookkeeper.volumes.index volume for a dedicated index mount, including PVC/volumeClaimTemplates support and StorageClass generation when needed.
• Updated values.yaml with documentation and examples for bookkeeper.volumes.index and bookkeeper.indexDirectories.

Verifying this change

• Make sure that the change passes the CI checks.

[Denovo1998]

@lhotari PTAL.